A Study on the Application of Non-azeotropic Refrigerant Mixtures in High Temperature Heat Pumps

与传统的单组分制冷剂相比，混合工质在高温热泵中的应用可以获得更高的性能系数、更低的冷凝压力和更高的可靠性。但众所周知，与单组分制冷剂相比，由于传质效应，二元混合工质的沸腾和冷凝换热系数都有所降低。为了研究换热系数降低对热泵性能的影响，建立了沸腾换热和冷凝换热传质影响的简化模型。针对沸腾换热和冷凝换热过程中，气液界面附近的质量扩散层导致换热阻力增大的问题，搭建了由水冷或加热式逆流换热器和开式压缩机组成的热泵实验台，以验证上述理论和研究混合工质的可行性。对纯工质R12和R22以及R22与R114在不同组成下的COP和换热性能进行了测试。在实验条件下，沸腾换热系数的降低比冷凝换热系数的降低更严重。换热系数的理论计算结果与实验结果的对比令人满意。当换热器的换热面积足够大，冷凝器处散热器流体的温升较大时，COP可以增加。

In comparison with conventional single-component refrigerants, the application of refrigerant mixtures in high temperature heat pumps may result in a higher coefficient of performance, lower condensing pressure, and higher reliability. But it is well known that the boiling and condensation heat transfer coefficients of binary mixtures are reduced due to the mass transfer effects compared with single-component refrigerants. Main purpose of this study is to evaluate the effect of the reduction in heat transfer coefficients on the heat pump performance.Simple models for evaluating the mass transfer effects on boiling heat transfer and condensation heat transfer were developed. In both boiling and condensation heat transfer, mass diffusion layer near the gas-liquid interface causes the increase in heat transfer resistance.In order to confirm the theories mentioned above and to investigate the feasibility of refrigerant mixtures, a heat pump test rig was constructed, which consisted of water-cooled or -heated counterflow heat exchangers and an open type compressor. The COP and the heat transfer performance for the pure refrigerants R12 and R22 and the mixtures R22 with R114 in different compositions were measured. Within the experimental conditions, the reduction in the boiling heat transfer coefficients was more serious than the condensation heat transfer coefficients. The comparison between the theoretical and experimental results of heat transfer coefficients was satisfactorily good. When the heat transfer area of heat exchangers is large enough and the temperature increase of the heat sink fluid at the condenser is large, the COP can increase.